1. Field of the Invention
The present invention relates generally to intervertebral disc devices suitable for delivery into a body to treat injured discs or to improve the function of non-injured discs.
2. Background
Methods of encapsulating a new intervertebral prosthesis are not known in the art. The most relevant prior art can be found in references concerning the encapsulation of stents and grafts in flexible biocompatible materials. However, these references are limited to teaching the coating of certain cylindrical structures with a single or double-sided coating of expanded polytetrafluoroethylene. These teachings do not address some or all of the requirements of a new intervertebral disc device which may be semicircular in cross-section, collapsible, fatigue resistant, biocompatible, and suitable for the dynamic disc environment.
Fluoropolymer tubings and other extruded or expanded polymer products are well known in the art and can provide an excellent material for encapsulation. Expanded polytetrafluoroethylene (ePTFE) is made by expanding polytetrafluoroethylene (PTFE) tubing or sheet material under controlled conditions during the manufacturing process. This process alters the physical properties of the material by creating microscopic pores in the structure of the material. The resulting material is imparted with unique physical properties that make it ideal for use in medical devices, electronic insulators, high performance filters, and a host of other applications. Material made from ePTFE differs from regular PTFE in that the material is microporous, soft, very flexible, has a lower dielectric constant, increased linear strength, and improved biocompatibility.
The structure of ePTFE is unique in that the material is made up of a number of solid nodes inter-connected by a matrix of thin fibrils. The gaps, or pores, between the fibrils are what allows the material to excel in applications requiring cellular ingrowth. ePTFE is also utilized for its endothelization and thrombogenic properties. Both ePTFE and PTFE resin have long been utilized for implantable medical devices due to their inertness and biocompatibility. The amount of expansion in ePTFE is usually referred to as internodal distance (IND). IND is a measure of the average distance between the material's nodes. ePTFE is commonly available from manufacturers in IND sizes ranging from 5μ to over 200μ. IND plays a role in the softness, flexibility, and degree of potential ingrowth that the material exhibits. Some commercial suppliers of ePTFE sinter the outside of the material after the expansion process in order to make it more durable and smooth.
Other useful PTFE based materials known in the art are PTFE resin aqueous dispersions or “aqueous PTFE” and heat shrinkable PTFE. Aqueous PTFE is commonly comprised of milky white dispersions of PTFE particles in water stabilized by wetting agents. Its composition is usually 30 to 60 percent PTFE particles and the remainder is comprised of water and a nonionic surfactant. Objects can be dip-coated with repeated passes into aqueous PTFE until the final desired thickness is obtained. The water can then be removed and the other ingredients deactivated. The melting temperature of these resins are around 325° Celsius. The dried resin particles can be coalesced by heat into a continuous coating or the substrate can remain coated, or impregnated, with unmelted particles. Both glass fabric and synthetic substrates can be processed.
Heat shrinkable PTFE tubing is well known in the art and provides a method for application of a tight, protective covering to items that will be subjected to the extremes of heat, corrosion, shock, moisture, and other critical environmental conditions. Shrinkable PTFE tubing requires around 340° Celsius to recover. Upon reaching approximately 330° the PTFE goes into the “gel” state (amorphous from crystalline). PTFE begins to shrink at 340° and completes its recovery during the cooling cycle.
This disclosure utilizes particular orthopedic references, nomenclature, and conventions. Accordingly, several background figures and descriptions are included to aid in the understanding of the environment under which specific embodiments of the invention may be used. FIGS. 1A and 1B show the general anatomy of a functional spine unit 345. In this description and the following claims, the terms ‘anterior’ and ‘posterior’, ‘superior’ and ‘inferior’ are defined by their standard usage in anatomy, i.e., anterior is a direction toward the front (ventral) side of the body or organ, posterior is a direction toward the back (dorsal) side of the body or organ; superior is upward (toward the head) and inferior is lower (toward the feet).